Microbial uptake of atmospheric methane from subterranean air in tropical karst

Mesocosm experiment in Minh Châu cave, Cát Bà island, Hải Phòng, Vietnam (November, 2015).

Sources and sinks of methane (CH4) are critical for understanding global biogeochemical cycles and their role in climate change. A growing number of studies have reported that CH4 concentrations in cave ecosystems are depleted, leading to the notion that these subterranean environments may act as sinks for atmospheric CH4. Recently, it was hypothesized that this CH4 depletion may be caused by radiolysis, an abiotic process whereby CH4 is oxidized via interactions with ionizing radiation derived from radioactive decay. An alternate explanation is that the depletion of CH4 concentrations in caves could be due to biological processes, specifically oxidation by methanotrophic bacteria.

Anrdt, Dương and Hướng preparing to pump methane biogas in Hanoi suburb for experiments in Cát Bà and Đồng Văn’s cave

We theoretically explored the radiolysis hypothesis and conclude that it is a kinetically constrained process that is unlikely to lead to the rapid loss of CH4 in subterranean environments. We present results from a controlled laboratory experiment to support this claim. We then tested the microbial oxidation hypothesis with a set of mesocosm experiments that were conducted in two Vietnamese caves.

Mesocosm experiment in Minh Châu cave, Cát Bà island, Hải Phòng (November, 2015).

Our results reveal that methanotrophic bacteria associated with cave rocks consume CH4 at a rate of 1.3–2.7 mg CH4 · m−2 · d−1. These CH4 oxidation rates equal or exceed what has been reported in other habitats, including agricultural systems, grasslands, deciduous forests, and Arctic tundra. Together, our results suggest that depleted concentrations of CH4 in caves are most likely due to microbial activity, not radiolysis as has been recently claimed. Microbial methanotrophy has the potential to oxidize CH4 not only in caves, but also in smaller-size open subterranean spaces, such as cracks, fissures, and other pores that are connected to and rapidly exchange with the atmosphere. Future studies are needed to understand how subterranean CH4 oxidation scales up to affect local, regional, and global CH4 cycling.

Graphical abstract for a publication in Chemical Geology. 
Equipment is being calibrated prior to entering Rồng cave, Đồng Văn, Hà Giang (December
The motion of hydrogen-filed balloons is used to measure the wind speed and wind direction in Rồng cave, Sảng Tủng commune, Đồng Văn, Hà Giang (December, 2015).
Prior walking to Lũng Táo cave, Đồng Văn, Hà Giang (December, 2015).
Arndt and Duong use a SARAD RTM 2200 gas analyzer to continuously record trace gas concentrations as they exit from Lung Tao cave during poor weather conditions (Dong Van, December 2015).
  • Nguyễn-Thuỳ, Dương, A. Schimmelmann, Hướng Nguyễn-Văn, Agnieszka Drobniak, Jay T. Lennon, Phương Hòa Tạ, Nguyệt Thị Ánh Nguyễn, 2017. Subterranean microbial oxidation of atmospheric methane in cavernous tropical karst. Chemical Geology, Volume 466, 5 September 2017, Pages 229-238 [https://doi.org/10.1016/j.chemgeo.2017.06.014]  [Graphical abstract JPEG]
  • Lennon, Jay T., Dương Nguyễn-Thùy, Trang Minh Phạm, Agnieszka Drobniak, Phương Hòa Tạ, Ngọc Ðức Phạm, Thomas Streil, Kevin D. Webster, A. Schimmelmann (2017) Microbial contributions to subterranean methane sinks. Geobiology 15 (2), 254-258. http://dx.doi.org/10.1111/gbi.12214
  • Schimmelmann, A., Jay T. Lennon, Duong Nguyen-Thuy, Phuong Ta Hoa, Agnieszka Drobniak, Kevin D. Webster, and Minh Schimmelmann (2016) Vietnam’s tropical karst is a sink for atmospheric methane greenhouse gas. 5th International Conference on Earth Science & Climate Change, July 25-27, 2016, Bangkok, Thailand. http://www.omicsonline.org/abstract/vietnam-s-tropical-karst-is-a-sink-for-atmospheric-methane-greenhouse-gas/
A large chamber in Na Luong cave (Ha Giang province, December 2015).
Jian, Arndt and Minh having a light lunch in Na Luong cave (December 2015).
Anrdt presented his research at the 5th International Conference on Earth Science & Climate Change, July 25-27, 2016, in Bangkok, Thailand.

Geosciences are improving the lives of people in Vietnam